The invention relates to a receiver and a method for CDMA transmission with enhanced path searcher as described in the subject of the independent claims.
In terrestrial communication the transmitted signal is reflected and refracted by a variety of smooth or rough terrains, so that it is replicated at the receiver with several time delays. Each individual path also arrives at is own amplitude and carrier phase. Propagation characteristics are qualitatively the some for all structures of signals though they will vary quantitatively with carrier frequency and terrain characteristics. The structures of the individual propagation paths can be identified and possibly exploited only on the extent that they can be distinguished from one other. In particular spread spectrum signals employ pseudo random sequences with chip time Tc inversely proportional to the spreading bandwidth. In this case the individual paths can be distinguished if they mutually separated by delays greater than Tc for then the various delayed versions of the signal will be mutually uncorrelated.
The path amplitudes will depend on the relative propagation distances and the reflective or refractive properties of the area. However in many cases particularly in confined areas each of the distinguishable multi path components will actually be itself the linear combination of several indistinguishable paths of varying amplitudes. To exploit energy in the multiple components of multi path propagation they must be identifies and acquired. It is particularly important to determine the relative delays and subsequently when possible their amplitudes and phases. This can be performed even with fully modulated signals, but the estimate is much precise and resulting performance is much improved if the path identification and parameter estimation is performed on an unmodulated signal. Unmodulated segments can be inserted every so often in the modulated signal particularly with time division multiplexing. However in spread spectrum systems it is much more effective and easier to separate the unmodulated pilot signals from the data modulated signal by assigning it on individual pseudo random sequence.
A pilot sequence for determining multi path component characteristics is well justified for one-to-many transmission channels such as the forward down link from a base station to multiple users. The optimum demodulator structure for a L multi path propagation channel is known as Rake receiver. Each multi path component demodulator is called a xe2x80x9cfingerxe2x80x9d of the rake. The pilot sequence tracking of a particular demodulator is started by time delay estimation of a given path as determined by the pilots sequences searcher. The demodulator forms the weighted phase-adjusted and delay-adjusted sum of L components.
In prior art the profile of the powers of each of the L paths is taken by checking the pilot sequence of one pilot channel on a slot by slot basis. This power profile is computed by noncoherent averaging of instantaneous channel profiles performed on this slot by slot basis. So the demodulator has to wait for the next pilot sequence in the next time slot to get more information to optimize the power profile.
For downlink mode the performance of this solution depends extremely on the signal to noise ratio of the pilot sequence. This means that the result of the demodulation in the receiver depends on the pilot sequence length itself and the distortions in the dedicated channel.
Moreover for high bitrate the correlation length is shorter because of the lower spreading factor of the signal. Thus for different bit rates performances of the path selection algorithm can be different. Within a CDMA receiver the selection of paths and delays of the transmission channel is needed in order to perform the channel estimation and data detection in presence of noise and interference. In a mobile terminal two different types of down link channels should be received: the dedicated channel DPCH carrying out user specific data and control information and common down link channels such as primary and secondary common control physical channels CCPCH, common pilot channel CPICH as well as synchronization channels. In principle the propagation conditions experienced by these two types of channels could be different, for example when the dedicated channels use antenna arrays that are not applicable for common channels. It should be noted that these common down link channels should not be used for channel estimation but only for path delay estimation. In fact fading processes are not the same on common control channels and dedicated channels when the latter are employing antenna array techniques.
In general the selection of channels paths is realized thanks to known pilot sequences included in each time slot of the considered physical channel.
The path searcher need a power profile prior to the path selection operation. The power profile might be computed by noncoherent averaging of instantaneous channel profiles performed on a slot by slot basis. An adaptive thresholding is also applied on the power profile, to try separating the true propagation channels encountered. The classical solution is to perform independently path delay selection for dedicated and common channels as shown in FIG. 2.
This solution has two main drawbacks. The structure is complex because of doubling the whole structure. The performance of second path searcher (3b) is decreased by a poorer signal to noise ratio of dedicated channels in comparison to common channels that are transmitted at a sufficient level in order lo be well received by all mobile terminals within the cell.
To obtain a better performance of the second path searcher (3b) and at the same time reduce the computational complexity the proposed solution is to perform the two path searching operations in a joint manner.
The invention as described below increases performance of a demodulator in a CDMA receiver due to a cascading of path searchers and so to get an optimized channel profile.
The improvements are done by:
A receiver for enhanced performance for COMA radio transmission comprising a Rake receiver (1) with a path searcher (3), a channel estimator (4) and a combiner (5) for selecting optimal channels paths where the path searcher (3) selects a set of path delays (7), characterized in that at least two path searchers (3a,3b) are connected in way that the output of the first path searcher (3a) is connected to one input of the second path searcher (3b) that input limits the range of selection.